Geoscience Reference
In-Depth Information
substantial areas of shelf in the Arctic). A more detailed comparison between shelf and
ocean production rates shows that the shelf seas have an average carbon fixation rate per
unit area a factor of
2.5 greater than in the deep ocean.
The abundant supply of phytoplankton provides the primary food source for the
rich fisheries of the shelf seas. Globally the shelf seas are the dominant source of fish
caught by humans; it has been estimated that over 90% of global fish catches come
from the shelf seas and adjacent upwelling area over the continental slope (Pauly
et al., 2002 ). Also, primary production involves the drawdown of CO 2 from the
atmosphere and the subsequent removal of some of the fixed carbon into the deep
ocean. Again, despite their relatively small surface area, current best estimates are
that the shelf seas are responsible for about 47% of the global annual export of
particulate organic carbon (Jahnke, 2010 ). This high proportion of carbon export is
partially a result of high primary productivity in the shelf seas, but it also arises
because of the unique physical environment in shelf seas and at the shelf edge.
The processes governing the shelf edge, and their biogeochemical and ecological
consequences, are the subject of Chapter 10 .
1.2
Economic value versus environmental health
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We depend on the shelf seas not only for fisheries but for a wide range of benefits.
The sediments of the shelf seas have been a major source of hydrocarbons, both oil
and gas, and are also widely exploited as a source of aggregates for building. We use
the shelf seas extensively for transport and for recreation, as many of us sail in them
and spend our holidays on their shores. We place a high value on the coastal marine
environment but, in many cases, compromise its health by using our shelf seas
and estuaries as low-cost dumping grounds for our domestic and industrial wastes.
The pressures on the shelf seas from human activity are the more acute because so
many of us live close to the coast. Approximately 40% of the human population
is located within 100 km of the sea, and coastal zones are host to much of our
industrial activity. Even where direct disposal of wastes by dumping and through
sewage pipelines has been stopped, rivers still carry large quantities of nutrients and
pollutants from terrestrial agriculture and industry into the shelf seas where they have
their first, and usually largest, impact. An excess of nutrients entering the shelf seas can
have seriously adverse effects in producing nuisance blooms of phytoplankton which
may themselves be toxic, or can lead to hypoxia (oxygen depletion) as they decay with
consequent disruption of the ecosystem and the mass mortality of marine organisms.
Heavy metals and organic pollutants tend to become concentrated as they progress up
the food chain and may have toxic effects in higher trophic levels including humans.
In addition to these pressures through inputs from industry and agriculture, the
welfare of the shelf seas is further threatened by the over-exploitation of fish stocks. It
has been estimated that fishing pressures in the North Sea are such that
25% of the
total North Sea biomass is removed each year (McGlade, 2002 ), much of it
by trawling which also ploughs up the seabed. Such intensive fishing can result in
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